Joe Biden has warned there is a “clear sign” Vladimir Putin is preparing to use chemical or biological weapons in Ukraine.

       The US president claimed Putin’s “back is against the wall” amid fierce Ukrainian resistance and the Russian leader is now resorting to “false flag” attacks as an excuse to unleash the banned weapons.

       Mr Biden also warned Russia could be preparing to launch cyberattacks against critical infrastructure as the war in Ukraine continues.

       “Putin’s back is against the wall,” he told American business leaders late on Monday. “He wasn’t anticipating the extent or the strength of our unity. And the more his back is against the wall, the greater the severity of the tactics he may employ.

       “Now he’s talking about new false flags he’s setting up, including he’s asserting that, we, in America, have biological as well as chemical weapons in Europe – simply not true.

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       “They’re also suggesting that Ukraine has biological and chemical weapons in Ukraine. That’s a clear sign he is considering using both of those.

       “He’s already used chemical weapons in the past, and we should be careful of what’s about to come. He knows there’ll be severe consequences because of the united Nato front, but the point is: It’s real.”

       Last week, the US president said he thought Putin was a “war criminal” and “murderous dictator” for his invasion of Ukraine.

       Joe Biden discusses the United States' response to Russian invasion of Ukraine and warns CEOs about potential cyber attacks from Russia at Business Roundtable's CEO quarterly meeting in Washington DC. REUTERS/Leah Millis

       (REUTERS)

       UK business minister Paul Scully on Tuesday told Sky News any use of chemical or biological weapons by Russian forces in Ukraine would “undoubtedly” be a war crime.

       It comes after it was alleged that hypersonic missiles, whose high velocity makes them extremely difficult to intercept, were used to destroy a weapons depot in Ukraine on Friday.

       Russian forces were initially arrayed for a high-speed blitz through Ukraine, aimed at quickly conquering the capital of Kyiv, but they have encountered stiff resistance.

       The Russian advance has “largely stalled on all fronts”, the UK’s Defence Intelligence agency said last week. Almost 10,000 Russian soldiers have been killed since the invasion began, according to official figures.

       Instead of rapidly conquering the country, Russia has instead begun a campaign of brutally shelling cities across Ukraine.

       On Monday, Ukraine rejected an offer to surrender the besieged port city of Mariupol.

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       In addition to the stalled military effort, Russia is facing devastating economic consequences at home, following sweeping sanctions from the US and others.

       Russian companies that rely on imported components, like carmaker Lada, have already seen production go idle, while the rouble has “fall through the floor,” according to Jeff Schott of the Peterson Institute for International Economics.

       “Interest rates are high,” he told The Washington Post. “Inflation is soaring. Imported goods are basically hard to find and are not being restocked because nobody is selling to Russia for fear that they will not get paid – or only paid in rubles.”

       Additional reporting by Associated Press